Complex or compound type springs have been proposed for holding bi-fold doors in the closed position or in the open position, and with two stable positions are known for use with box lids and the like.
U.S. Pat. No. 3,849,835 issued on Nov. 26, 1974 to Torsti T. T. Jerila shows one of the complex type; examples of other such hinges are shown in the following U.S. Pat. Nos.:
2,290,219 to S. E. H. Ulfving, 7-21-42, shows the principle of multi-position (in this case 3-way) stability; PA1 515,334 issued to A. S. Held on 2-27-1894, shows a hinge form having an arcuate portion and cross arms; PA1 2,541,377 issued to G. W. Neely on 2-13-51, shows a hinge-related spring assembly that snaps doors open and closed at positions 180.degree. apart; PA1 2,998,618 issued to L. T. Roy on 9-3-61, shows another arcuate spring arrangement for hinged closures; PA1 4,452,373 issued to D. J. and R. J. Pearce on 6-5-84, showed another form of "C"-shaped spring and engagement with pivoted closure. PA1 is suited for wood or for metal doors; PA1 is compact so that the exposed portion of it can lie between the top of the door and almost any header; PA1 extends little or not at all beyond the thickness of the door in any position of use; PA1 requires a minimum of installation work, normally only laying out of two holes to generous tolerances, drilling same parallel with each other, and inserting the spring assembly ends in the holes; PA1 tends to self-align in use even though not perfectly installed; PA1 is longwearing and protects wooden doors from mounting-hole enlargement due to wear over periods of time; PA1 is independent of, but acts in conjunction with bifold door hinges; PA1 is light weight and economical, and that is versatile and size-adaptable to suit most bifold doors.